Nowadays, direct current to direct current (DC/DC) converters are widely utilized to satisfy the power supply requirements of electronic applications. In some applications, DC/DC converters generally utilize the trench MOSFET as a high efficiency switch.
In conventional synchronous DC/DC converter circuits, to avoid shoot-through current damaging the MOSFET, it is forbidden to turn on the primary MOSFET and the synchronous rectifier MOSFET at the same time. Before either MOSFET is turned on, both of the MOSFETs need to be turned off first. This period of turning off both MOSFETs is called dead time. During the dead time, current is able to flow through the parasitic PN diode inside the trench MOSFET. However, the parasitic diode has a relative high forward voltage drop (approximate 0.7V), which reduces the efficiency of the converter. Moreover, Since PN diode is a type of minority-carrier device, the backward recovery characteristic of PN parasitic diode is relative poor.
Compared with PN diode, Schottky diode has a smaller forward voltage drop because of the metal-semiconductor contact structure. Also Schottky diode is a type of majority-carrier device, so it has better backward recovery performance. With these advantages, Schottky diode is integrated into MOSFET and parallel-coupled with the PN parasitic diode, configured to achieve lower power consumption and higher switching speed.
Generally, a trench MOSFET integrating Schottky diode has such a structure that a completely diffused p-type well or an extra deeper p-type well beside Schottky diode area pinches off this area under low drain voltage. But for completely diffused p-type well, a large area of die may be taken up for the big size p-type well due to its deep junction. For an extra deeper p-type well, additional masks and process steps may be added. Moreover, both of these devices have low die size utilization rates. Thus, an improved device is desired.